Controlling pine and oak processionary caterpillars in urban areas: ANSES recommends combining preventive and curative measures

The caterpillars of pine and oak processionary moths weaken their host trees by eating their needles or leaves. The caterpillars also affect human and animal health, as their urticating bristles cause extreme itching and allergic reactions resulting in a form of dermatitis known as erucism. 

The caterpillars are found in different regions: the pine processionary is mostly found around the Mediterranean Basin and the Atlantic seaboard as far as the Pyrénées-Atlantiques département, while the oak processionary is commonest in the Alsace, Bourgogne, Ile-de-France, Centre, Poitou-Charentes and Midi-Pyrénées Regions.

As the species of tree on which they feed are found in urban areas where it is impossible to apply airborne insecticide treatments, the Ministries of Health, Labour and the Environment requested ANSES to review the possible alternatives (physical, chemical, etc.) for combating this threat in urban areas, while preserving the health of the local population and the environment.

To carry out this work, the Agency instructed its Expert Committee on Biological risks for plant health to carry out an expert appraisal, assisted by an ad hoc group of rapporteurs.

The work of the Agency 

The appraisal found that there is currently no biocidal⁽¹⁾  insecticide authorised for use against pine and oak processionary caterpillars. However, there are certain plant protection products available for combating these caterpillars for plant health purposes that can be applied from the ground.

The most effective strategy for combating these caterpillars in urban environments involves a combination of preventive and curative methods, in association with surveillance measures. The intensity of the intervention needs to be adjusted depending on the compartments of the urban environment concerned, distinguishing between: the application of methods aiming to eradicate the caterpillar in busy areas (schools, pedestrian areas, etc.) and the application of methods aiming to keep the caterpillar population down to levels compatible with their use in less busy areas (boulevards and large public parks).

For oak processionary caterpillars

Current knowledge of the biology and epidemiology of the oak processionary caterpillar seems too fragmentary to allow any recommendations for its control. A considerable research effort must therefore be undertaken to establish a scientific and technical basis on which to develop alternative and integrated methods for managing the risk associated with the oak processionary. 

For pine processionary caterpillars

For the purposes of its expert appraisal, the Agency analysed two cases: a “zero tolerance” case, in which local constraints (school playgrounds, protected trees, high-traffic public parks, tree-lined avenues of great importance for tourism, etc.) require the eradication of processionary populations, and a case in which the presence of low caterpillar population levels might be acceptable and understood by local residents (roadside trees, large urban public parks, etc.). 

For zero tolerance situations

The Agency considers that, as a preventive measure, it would be better not to plant the more vulnerable tree species or those attractive to pine processionary caterpillars or to plant them in copses in association with deciduous species with the same rapid growth (e.g. birch). Checks should also be made to ensure that there are no chrysalises (the pupal stage between the caterpillar and adult stages of moths) in the earth in the containers for trees brought from nurseries for planting; alternatively, the earth in the containers may be treated. 

Where trees are already in place, measures should be taken in summer to detect whether pine processionaries are present with the use of pheromone traps (traps using chemical substance comparable to hormones, which attract insects). If males are found in the traps, several curative measures can be combined to combat the threat. If the trees are not too numerous or tall and access is easy, the nests can be destroyed physically and insecticide can be applied from the ground in compliance with plant protection regulations. In all such cases, caterpillar traps (sticky bands round the tree trunks) can be installed before the start of the procession activity to avoid the risk of urtication caused by caterpillars on the ground. The association of these two alternative trapping techniques, the adults in summer and the caterpillars in winter and spring, should provide an adequate response to this requirement for zero tolerance when the instructions for installation are properly observed and levels of infestation are non-epidemic.

For situations where the aim is reduction of population levels

The Agency recommends favouring preventive methods, but these can be of limited efficacy. The forest floor and the edges of pine woods should not be left bare but covered with dense vegetation (shrubs). Landscape specialists should avoid unmixed pine plantations and design their parks and avenues with an assortment of conifers and deciduous species, particularly fast-growing species such as birches or willows, which are repulsive to the pine processionary moth and also provide shelter and a wider and consequently more effective range of beneficial fauna. Biological pest control can be reinforced by installing nest boxes for insect-eating birds (hoopoes or tits) and shelters for bats.

The Agency also recommends monitoring population levels of the pine processionary with the use of pheromone traps in summer and by counting the nests in winter.

Lastly, any curative methods used in this context should be repeatable every year to ensure a long-term effect. This includes the physical destruction of the nests along with mass trapping, sexual confusion techniques, or the use of pine processionary mot repellents in the case of isolated trees or parks. In areas with high levels of infestation, insecticide treatments may be applied from the ground.

⁽¹⁾ Biocidal products are formulations of active substances intended for household or industrial use. These common, everyday products include household disinfectants, insecticides and other products whose purpose is to eliminate, destroy or deter harmful organisms (moulds, bacteria, viruses). The active substance present in the biocidal product can be a chemical compound or derived from a micro-organism that exerts its biocidal action on or against harmful organisms.